Muto vs. Perdo in spell design?

My GM and I are having a bit of a time trying to figure out how to represent a spell I'm trying to create. The spell would turn a human's skin into glass; obviously, a MuCo spell with a Ter requisite. Mechanically, it's the inverse of Gift of the Bear's Fortitude (Pg 131, ArM5), but with a penalty to defense instead of the penalty to fine manipulation. Though, I could potentially see keeping that as well.

The problem is that my GM feels it should have a Pe requisite as well, since Perdo covers weaken as well as destroy. My objection is that Perdo seems to be focused on inflicting wounds and fatigue levels, not giving penalties to things. Conversely, the fluff for Muto Corpus explicitly mentions hindering spells that are cast on foes, and contain the only instances I can find of applying penalties to enemies (it's often paired with a bonus, but I'll take what I can get). What is the forum consensus on the matter? Can Muto apply penalties by itself without Perdo?

Perdo is not required. It is the difference between transformation and destruction/decay. In no way would flesh ever "decay" to glass. And the effect of changing to glass is not inherently destructive, rather it is then what happens to the body.

It is a very unnatural transformation too, so should be high magnitude.

Well, the base effect of turning someone into an inanimate object is a base level 25 effect. Bumping the duration up to Sun and the range to Voice makes it a rank 40 effect. If it's meant to last for just one combat, going Diameter instead of Sun might work and would take the rank down to 35 instead. Part would make sense for the Target, but Gift of the Bear's Fortitude is only Individual, and that's basically the spell I want to make, but in reverse. So yeah, the rank is sorta high, but not impossible, especially since my mage's best three arts are Muto, Terram, and Corpus.

I feel that burdening such spells with Perso requisites is a remnant of the 1st and 2nd editions. Brittle skin is unnatural and smells like Muto IMHO. But if we're talking about affecting a sword so the metal becomes brittle I could see using Perdo for this, but by itself instead of Muto. Because such a thing can happen to metal.

OTOH is a spell just needs to give a human target worse Soak, then wouldn't PeCo be used to reduce his Stm? after all Cr Co is used to increase it???

I want to avoid using Perdo as much as possible; my character is a Merinita with the Harmless Magic flaw. I also envisioned the victim having lower Defense as well, but it would mostly reduce their soak. While sickening and decaying the target's body, reducing Stamina with Perdo Corpus could achieve the same effect, that's not the path that I want to take.

Not a chance. End of story. The spell is not destroying anything.

100% yes.

There´s a problem with this though, glass on top of a supporting surface(the body in this case) is actually quite damage resistant. If a person under the spell can still move, this means the glass has some degree of stretch ability, which means the inherent brittleness of glass becomes mostly irrelevant, and instead the thing that will be most notable is how strong glass is.
In short, glass that you reduce the risk of shattering for, is very strong for its weight.

So, either you make the glass be rigid, also preventing the target from moving unless breaking out(which becomes seriously gory and nasty alltogether) or the glass shatters(giving the word gory a whole new dimension), or you specify that the kind of glass you create is of the thin and very weak sort.
Ingame mechanichally, it doesn´t need to be any difference unless you want it, but you may need to know it to avoid aguments.